1. Field of the Invention
The present invention generally relates to a transmission electron microscope. More specifically, the invention is concerned with a transmission electron microscope equipped with an energy filter for spectrally separating only electrons that have specific energy from those transmitted through a specimen to thereby make it possible to obtain an element distribution or map image of a minute or fine region. The invention is further concerned with a method of observing element map or distribution by using the transmission electron microscope equipped with the energy filter.
2. Description of the Related Art
The transmission electron microscope is an apparatus for magnifying a specimen image for observation by using an electron beam and electron lenses and is employed for identifying a fine structure of a specimen. On the other hand, the energy filter is a device for spectrally filtering or separating the electron beams transmitted through the specimen for thereby extracting only the electrons having a specific width of energy. By combining the transmission electron microscope with the energy filter, there can be realized a transmission electron microscope system which allows an image formed only by electrons having a specific energy width.
In the transmission electron microscope equipped with the energy filter for which adjustment of the optical axis has been completed, it is possible to obtain an electron image (zero-loss image) formed by only those electrons which have undergone elastic scattering by inserting an energy selecting slit on the optical axis. By increasing the acceleration voltage for the incident electron beam by .delta.E for observation, electrons having lost energy by .delta.E upon transmission through a specimen are caused to pass through the energy selecting slit after having passed through the energy filter. Accordingly, energy-filtered image formed by electrons having lost the energy by .delta.E can be obtained by increasing the acceleration voltage by .delta.E.
Electrons having transmitted through a specimen lose energy due to inelastic scattering, as exemplified by plasmon loss and core loss, to exhibit an energy spectrum. The core-loss energy is a quantity inherent to elements constituting the specimen. Accordingly, the transmission electron microscope image formed by electrons undergone a specific energy loss exhibits two-dimensional distributions corresponding to elements constituting the specimen, respectively. However, the energy loss due to the inelastic scattering spreads over a wide energy range, giving rise to overlap of information of other element(s) as the background. Unless the background is separated to be eliminated, there can be obtained no intrinsic elemental distribution or map image. For separation and elimination of the influence of the background to thereby obtain a distribution or map image of specific elements, there have heretofore been proposed two types of methods among others, which will be mentioned below.
In a first method, two images in total are used, i.e., an energy-filtered image obtained by providing an energy window in a region of core-loss energy and an energy-filtered image obtained by providing an energy window at a region immediately preceding to the region of core-loss energy for suppressing the influence of core-loss electrons. According to this method, the two images mentioned above are inputted to a computer with the aid of an image pickup device such as a television camera. By regarding the second mentioned image as the background for the first mentioned image, inter-image subtraction processing is executed within the computer for subtracting the second mentioned image from the first mentioned image, to thereby separate the background for eliminating the influence thereof. In this way, a two-dimensional distribution of specific elements can be obtained.
In a second method, there are used three images in total. More specifically, in addition to the two energy-filtered images used in the first method described above, another energy-filtered image is generated by providing an energy window in a region which contains no core-loss electrons and which differs from the regions mentioned previously in conjunction with the first method. According to the second method, the three images are inputted to a computer by means of an image pickup device such as a television camera, whereupon on the basis of the two images formed by electrons including no core-loss electrons, change of the background intensity for a change of energy is determined for all the pixels by the computer, and the accurate background intensity of the energy-filtered image formed by electrons including core-loss electrons is arithmetically determined or calculated for all the pixels of the image. By subtracting the background intensity determined in this manner, influence of the background is separated and eliminated, whereby a two-dimensional distribution or map image of specific elements can be obtained.
In the case of the first method, there exists difference between the background intensity used in the arithmetic operation and the intrinsic background intensity. Accordingly, although the arithmetic processing is simple, the first method is disadvantageous in respect to the quantativeness.
On the other hand, with the second method, it is certainly possible to determine the intrinsic background with high accuracy by using two images. However, because the arithmetic processing has to be performed for all the pixels of the image, a lot of time is taken for the calculation. Parenthetically, it is reported that the time taken for such calculation amounts to about one minute at the shortest even when a high-performance computer is employed (see Koji Kimoto, Tatsumi Hirano, Katsuhisa Usami, Naruto Sunakozawa and Toshitaka Taya: Proc. 50th Meeting of the Microscopy Society of Japan, (1994) 76). Such being the circumstances, feedback of the result of the processing in the course of experiment is rendered impossible.
In the case of the first method, the time taken for the arithmetic operation is relatively short when compared with the second method. However, difficulty will be encountered in an attempt for applying the first method for observation of a specimen in which distribution of elements of interest changes in continuation as the time lapses as well as a specimen which deforms progressively. Furthermore, for a specimen which is drifting, arithmetic operation for manipulation such as renewal position alignment will be required. Of course, use of a high-performance computer is undesirable from the economical viewpoint.
At this juncture, it should also be mentioned that the position and the width of the energy window are important factors for evaluating the quality of the image obtained finally and quantitativeness of the elemental distribution or map image. Accordingly, it is desirable that these factors can be set optimally during experiment. However, unless the real-time processing is possible, setting of such optimal conditions will have to rely on the experiences of a person conducting the experiment, presenting thus a problem which is difficult to solve technically. Besides, the conventional methods suffer a drawback that in the course of calculating the background, pixel-to-pixel errors will be involved due to noise components contained in the image, as a result of which the S/N ratio is degraded, giving rise to another problem.